The rt_sigqueueinfo() and rt_tgsigqueueinfo() system calls are the
low-level interfaces used to send a signal plus data to a process or
thread. The receiver of the signal can obtain the accompanying data
by establishing a signal handler with the sigaction(2)SA_SIGINFO
flag.
These system calls are not intended for direct application use; they
are provided to allow the implementation of sigqueue(3) and
pthread_sigqueue(3).
The rt_sigqueueinfo() system call sends the signal sig to the thread
group with the ID tgid. (The term "thread group" is synonymous with
"process", and tid corresponds to the traditional UNIX process ID.)
The signal will be delivered to an arbitrary member of the thread
group (i.e., one of the threads that is not currently blocking the
signal).
The uinfo argument specifies the data to accompany the signal. This
argument is a pointer to a structure of type siginfo_t, described in
sigaction(2) (and defined by including <sigaction.h>). The caller
should set the following fields in this structure:
si_code
This must be one of the SI_* codes in the Linux kernel source
file include/asm-generic/siginfo.h, with the restriction that
the code must be negative (i.e., cannot be SI_USER, which is
used by the kernel to indicate a signal sent by kill(2)) and
cannot (since Linux 2.6.39) be SI_TKILL (which is used by the
kernel to indicate a signal sent using tgkill(2)).
si_pid This should be set to a process ID, typically the process ID
of the sender.
si_uid This should be set to a user ID, typically the real user ID of
the sender.
si_value
This field contains the user data to accompany the signal.
For more information, see the description of the last (unionsigval) argument of sigqueue(3).
Internally, the kernel sets the si_signo field to the value specified
in sig, so that the receiver of the signal can also obtain the signal
number via that field.
The rt_tgsigqueueinfo() system call is like rt_sigqueueinfo(), but
sends the signal and data to the single thread specified by the
combination of tgid, a thread group ID, and tid, a thread in that
thread group.

EAGAIN The limit of signals which may be queued has been reached.
(See signal(7) for further information.)
EINVAL sig, tgid, or tid was invalid.
EPERM The caller does not have permission to send the signal to the
target. For the required permissions, see kill(2). Or:
uinfo->si_code is invalid.
ESRCH rt_sigqueueinfo(): No thread group matching tgid was found.
rt_tgsigqueinfo(): No thread matching tgid and tid was found.

Since these system calls are not intended for application use, there
are no glibc wrapper functions; use syscall(2) in the unlikely case
that you want to call them directly.
As with kill(2), the null signal (0) can be used to check if the
specified process or thread exists.

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 RT_SIGQUEUEINFO(2)